How twisting of molecules leads to twisting of liquid crystals.

What is it about?

Adding a tiny amount of chiral dopant induces an achiral uniaxial nematic liquid crystal to twist into a chiral helicoidal cholesteric phase. We argue that the origin of such helical twisting power lies in correlated fluctuations between right- and left-twisted conformations of molecules in the nematic phase and that molecular chirality is transferable. We also show that such switchable molecules can spontaneously segregate when cooled, forming coexisting right- and left-handed chiral domains. This kind behavior is observed, e.g., in a novel class of ferroelectric liquid crystalline materials, and typically explained in terms of a bent or banana shape of liquid crystal molecules. Our work explains why chiral segregation is possible even if molecules do not have an obvious bent shape, as often is the case.

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Photo by Steve Johnson on Unsplash

Photo by Steve Johnson on Unsplash

Why is it important?

Molecules that are not equal to their mirror images are called chiral, and have a definite handedness. How handedness of molecules in life on earth came about is very poorly understood, and requires self-sorting of molecules that started out in equal mixtures. The way in which handedness on the molecular scale leads to handedness on the observable, macroscopic scale is ideally studied in liquid crystals, as these are highly sensitive to external influence including doping by chiral molecules. Furthermore, control of chirality in liquid crystalline materials is key for applications in areas ranging from sensors, switches, filters and displays, to structural colors, e-paper and soft robotics, and recent years have because of this witnessed a surge of interest in chiral materials.

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